A communication system is known from the published European Patent Application EP 0515703 A1, wherein an exchange connects subscriber stations to a single conference unit. This communication system is used to carry out video conferences. The exchange establishes point-to-point channels between the subscriber stations and the conference unit. In such a communication system, the subscriber stations involved in a conference transmit signals, which in the present case are video and audio signals, to the single conference unit. These transmitted signals are mixed in the conference unit in accordance with the desired arrangement signalled by the subscriber stations. The conference unit then transmits the mixed signals to the respective subscriber stations.
Instead of the assumedly simplified single exchange, a communication system of this type usually has a communications network composed of a number of exchanges, which are interconnected. The subscriber stations involved in the conference are generally connected to different exchanges of the communications network. To transmit the signals from the respective subscriber stations to the single conference unit, each signal being transmitted requires a channel of a predetermined bandwidth in the communications network. The communications network therefore uses a large transmission capacity when it transmits the signals.
Electronic conferences, particularly broadband multimedia conferences, are gaining ever more significance. However, the costs of utilizing transmission capacities are still very high, thus a greater acceptance can only be expected when these transmission costs are lowered.